I've used boards from control resources. http://www.controlres.com They have all kinds of products for many temp control applications. I bought a board for $40 that uses PWM control and has 2 sensor inputs. It keeps a $35,000 LCS audio dsp rack nice and cool (that probably rivals the heat output of your average space heater) without excessive noise from fans running at full speed in my theater here in NY. They also have one with an alarm out if the rpm falls below a set threshold. Cool stuff. I actually have a tach control board I don't need from them that you could have if you're interested.

Those are nice boards. Unfortunately I want the fans to idle up to 55C, then ramp to full power at 65C. Most of these boards ramp to full power at much lower temperatures, and the adjustable ones are made for 24V or 48V fans.

This drawing from the Microchip datasheet seems like just the circuit I need, except I will use a 30K NTC thermistor instead of 10K.

Heh, I had the very same design issue when I bought one of the boards. One of the engineers said to just put a 100K or so resistor across the thermistor to "trick" the input sense.
That's a neat chip, just the kind of thing I was looking for 6 months ago.